(Assuming you do as I know a lot of people don't)
If you're writing to a couple or a family, whose name do you put first (inside the card, not on the address)?
I will put the name of the person I knew first, first and then the children by age.
DH and I have been together for ever and most people met us when we were a couple so we get a mixture (but since you ask, I'm 'winning' at the moment
)